[PATCH] KVM: SVM: document KVM_MEM_ENCRYPT_OP, let userspace detect if SEV is available

From: Paolo Bonzini
Date: Fri Mar 20 2020 - 13:42:52 EST


Userspace has no way to query if SEV has been disabled with the
sev module parameter of kvm-amd.ko. Actually it has one, but it
is a hack: do ioctl(KVM_MEM_ENCRYPT_OP, NULL) and check if it
returns EFAULT. Make it a little nicer by returning zero for
SEV enabled and NULL argument, and while at it document the
ioctl arguments.

Cc: Brijesh Singh <brijesh.singh@xxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
.../virt/kvm/amd-memory-encryption.rst | 25 +++++++++++++++++++
arch/x86/kvm/svm.c | 3 +++
2 files changed, 28 insertions(+)

diff --git a/Documentation/virt/kvm/amd-memory-encryption.rst b/Documentation/virt/kvm/amd-memory-encryption.rst
index d18c97b4e140..c3129b9ba5cb 100644
--- a/Documentation/virt/kvm/amd-memory-encryption.rst
+++ b/Documentation/virt/kvm/amd-memory-encryption.rst
@@ -53,6 +53,29 @@ key management interface to perform common hypervisor activities such as
encrypting bootstrap code, snapshot, migrating and debugging the guest. For more
information, see the SEV Key Management spec [api-spec]_

+The main ioctl to access SEV is KVM_MEM_ENCRYPT_OP. If the argument
+to KVM_MEM_ENCRYPT_OP is NULL, the ioctl returns 0 if SEV is enabled
+and ``ENOTTY` if it is disabled (on some older versions of Linux,
+the ioctl runs normally even with a NULL argument, and therefore will
+likely return ``EFAULT``). If non-NULL, the argument to KVM_MEM_ENCRYPT_OP
+must be a struct kvm_sev_cmd::
+
+ struct kvm_sev_cmd {
+ __u32 id;
+ __u64 data;
+ __u32 error;
+ __u32 sev_fd;
+ };
+
+
+The ``id`` field contains the subcommand, and the ``data`` field points to
+another struct containing arguments specific to command. The ``sev_fd``
+should point to a file descriptor that is opened on the ``/dev/sev``
+device, if needed (see individual commands).
+
+On output, ``error`` is zero on success, or an error code. Error codes
+are defined in ``<linux/psp-dev.h>`.
+
KVM implements the following commands to support common lifecycle events of SEV
guests, such as launching, running, snapshotting, migrating and decommissioning.

@@ -90,6 +113,8 @@ Returns: 0 on success, -negative on error

On success, the 'handle' field contains a new handle and on error, a negative value.

+KVM_SEV_LAUNCH_START requires the ``sev_fd`` field to be valid.
+
For more details, see SEV spec Section 6.2.

3. KVM_SEV_LAUNCH_UPDATE_DATA
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 91000501756e..f0aa9ff9666f 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -7158,6 +7158,9 @@ static int svm_mem_enc_op(struct kvm *kvm, void __user *argp)
if (!svm_sev_enabled())
return -ENOTTY;

+ if (!argp)
+ return 0;
+
if (copy_from_user(&sev_cmd, argp, sizeof(struct kvm_sev_cmd)))
return -EFAULT;

--
2.18.2